1. Field of the Invention
The present invention relates generally to earth boring bits used to drill a borehole for the ultimate recovery of oil, gas, or minerals. More particularly, the invention relates to rolling cone rock bits and to an improved cutting structure and cutter elements for such bits. Still more particularly, the invention relates to enhancements in cutter element shape, positioning and orientation in the drill bit.
2. Description of the Related Art
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by revolving the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or “gage” of the drill bit.
A typical earth-boring bit includes one or more rotatable cone cutters that perform their cutting function due to the rolling movement of the cone cutters acting against the formation material. The cone cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cone cutters thereby engaging and disintegrating the formation material in its path. The rotatable cone cutters may be described as generally conical in shape and are therefore referred to as rolling cones.
Rolling cone bits typically include a bit body with a plurality of journal segment legs. The rolling cones are mounted on bearing pin shafts that extend downwardly and inwardly from the journal segment legs. The borehole is formed as the gouging and scraping or crushing and chipping action of the rotary cones remove chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
The earth disintegrating action of the cone cutters is enhanced by providing the cone cutters with a plurality of cutter elements. Cutter elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as “TCI” bits, while those having teeth formed from the cone material are commonly known as “steel tooth bits.” In each instance, the cutter elements on the rotating cone cutters breakup the formation to form new borehole by a combination of gouging and scraping or chipping and crushing.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits which will drill faster and longer and which are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed depends upon its ability to “hold gage” (meaning its ability to maintain a full gage borehole diameter), its rate of penetration (“ROP”), as well as its durability or ability to maintain an acceptable ROP. The form and positioning of the cutter elements (both steel teeth and tungsten carbide inserts) upon the cone cutters greatly impact bit durability and ROP and thus, are critical to the success of a particular bit design.
The inserts in TCI bits are typically inserted in circumferential rows on the rolling cone cutters. Most such bits include a row of inserts in the heel surface of the cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to align generally with and ream the sidewall of the borehole as the bit rotates.
In addition to the heel row inserts, conventional bits typically include a circumferential gage row of cutter elements mounted adjacent to the heel surface but oriented and sized so as to cut the corner of the borehole. In performing their corner cutting duty, gage row inserts perform a reaming function, as a portion of the insert scraps or reams the side of the borehole, and also perform bottom hole cutting, a duty in which the gouges the formation material at the bottom of the borehole. This dual function of a gage row insert many times leads to design compromises for such insert.
Conventional bits also include a number of additional rows of cutter elements that are located on the cones in circumferential rows disposed radially inward or in board from the gage row. These cutter elements are sized and configured for cutting the bottom of the borehole, and are typically described as inner row cutter elements.
Earthen formations generally undergo two types of fractures when penetrated by a cutter element that protrudes from a rolling cone of a drill bit. A first type of fracture is generally referred to as a plastic fracture, and is the type of fracture where the cutter element penetrates into the rock and volumetrically displaces the rock by compressing it. This type of fracture generally creates a crater in the rock that is the size and shape of that portion of the cutter element that has penetrated into the rock.
A second principal type of fracture is what is referred to as a brittle fracture. A brittle fracture typically occurs after a plastic fracture has first taken place. That is, when the rock first undergoes plastic fracture, a region around the crater made by the cutter element will experience increased stress and will weaken and may crack in that region, even though the rock in that region surrounding the crater has not been displaced. This region of increased stress is generally recognized as the “Hertzian” contact zone. However, in certain formations, when the cutter element displaces enough of the rock and creates enough stress in the Hertzian contact zone adjacent to the plastic fracture, that rock in the region of increased stress may itself break and chip away from the crater. Where this occurs, the cutter element effectively removes a volume of rock that is larger than the volume of rock displaced in the plastic fracture. The characteristics of these fractures depend largely on the geometry of the cutter element and the properties of the rock that is being penetrated.
Because a brittle fracture removes more rock material than a plastic fracture, it would be advantageous to provide a cutter element suitable for inducing brittle fractures that would perform that function without requiring increased force or weight on bit. Thus, to increase a bit's rate of penetration (ROP), it is desirable to increase the bit's ability to initiate brittle fractures at the locations where the cutter element engages the formation material so that the volume of rock removed by each hit or impact of the cutter element is greater than the volume of rock actually penetrated by the cutter element.
A variety of different shapes of cutter elements have been devised. In most instances, each cutter element is designed to optimize the amount of formation material that is removed with each “hit” of the formation by the cutter element. At the same time, however, the shape and design of a particular cutter element is also dependent upon the location in the drill bit in which it is to be placed, and thus the cutting duty to be performed by that cutter element. For example, in general, heel row cutter elements are generally made of a harder and more wear resistant material, and have a less aggressive cutting shape for reaming the borehole side wall, as compared to the inner row cutter elements where the cutting duty is more of a gouging, digging and crushing action. Thus, in general, bottom hole cutter elements generally tend to have more aggressive cutting shapes than heel row cutters.
It is understood that cutter elements, depending upon their location in the rolling cone cutter, have different cutting trajectories as the cone cutter rotates in the borehole. Thus, conventional cutter elements have been oriented in the rolling cone cutters in a direction believed to cause optimal formation removal. However, it is now understood that cutter elements located in certain portions of the cone cutter have more than one cutting mode. More particularly, cutter elements in the inner rows of the cone cutters, particularly those closest to the nose of the cone cutter (and the center line of the bit), include a twisting motion as they gouge into and then separate from the formation. Unfortunately, however, conventional cutter elements, such as a chisel shaped insert, having a single primary cutting edge, are usually oriented to optimize the cutting that takes place only in the cutter's circumferential cutting trajectory, as the do not have particular features to take advantage of cutting opportunities as the cutter element twists.
Accordingly, to provide a drill bit with higher ROP, and thus to lower drilling costs incurred in the recovery of oil and other valuable resources, it would be desirable to provide cutter elements designed and oriented so as to enhance brittle fracture of the rock formation being drilled.